Hybrid systems of a type involving both analog and digital circuitry usually require some form of isolation between the analog and digital circuit sections, to prevent contamination therebetween as a result of ground loops or capacitive coupling. One example of a hybrid system of this type is a laboratory instrumentation calibrator, such as a Model 5100 Multifunction Calibrator manufactured by the John Fluke Manufacturing Company, which generates precisely controlled analog signals as calibration standards for instrumentation such as digital voltage meters. In this case, an analog circuit section of the hybrid system carries out measurement of very low amplitude analog input signals, conversion of the analog signals to digital signals for processing in the digital circuit section which usually includes a microprocessor and development of analog output signals in response to digital control signals developed by the digital circuit section.
To maintain a precisely controlled analog output signal, it is imperative that the analog and digital circuit sections be maintained electrically isolated from each other so that, for example, ground loop currents will not flow between circuit sections and high frequency components of digital signals will not be coupled to the analog section as noise, thereby raising the potential of the analog ground plane and inducing spurious components into the analog output signal. Similarly, isolation would prevent sharp changes in the analog signal from being coupled to the digital circuit section producing logical errors therein.
Guarded crossings allow the guarded sections to float at a different potential from the chassis of the instrument, which usually is at or near earth potential, and also serve to minimize both electrical and magnetic coupling between guarded and unguarded sections of an instrument. Three techniques for isolating the analog and digital circuit sections using "guarded crossings" are often employed.
A first type of guarded crossing is single ended transformer isolation, shown in FIG. 1, wherein digital signals are transmitted in serial form through the primary and secondary windings 10a, 10b of a pulse transformer 10 interfacing an analog circuit section 12 comprising analog input and output signal processing circuitry 12a, an analog-to-digital converter 12b and a driver 12c, and a digital circuit 14 driven by the analog circuit 12 through the transformer 10 and a buffer 14a. A similar pulse transformer (not shown) couples digital signals in the opposite direction, if bidirectional data transmission is required, e.g., to return reading or status information to the circuit 12. This technique is characterized by relatively low cost and low capacitive coupling; however, the transformer is susceptible to common mode voltage transients since the transformer is referenced to ground. Furthermore, voltage level shifting and detection that are required as a result of transformer isolation, particularly at high frequencies, increase cost and complexity of signal processing circuitry within the instrument.
A second type of guarded crossing is balanced pulse transformer isolation, shown in FIG. 2, wherein pulse transformer 16 has center-tapped primary and secondary windings 16a, 16b that are grounded, as shown, to present a balanced output to receiver 18 and thereby to reduce the effect of common mode voltage transients. One transformer couples a signal positively and the other signal is driven by an inverted signal, as shown in the Figure. On the receiving end, the transformers drive circuitry which recreate the original waveform. Zener diodes 18a, 18b at the receiving end minimize the response of pulse transients, and since the transformers are balanced, the circuit does not respond to common mode voltage transients. However, the cost of this type of guarded crossing is greater than that of single winding transformer isolation, and there is capacitive coupling between the primary and secondary windings which couples high frequency transients between the isolated circuits.
A third type is optocoupling, shown in FIG. 3, wherein optical coupler 20 consists of an LED and light responsive transistor sealed within an optically opaque enclosure 22 for coupling digital signals between isolated circuits through a universal asynchronous receiver and transmitter (UART) 23. Because optocouplers have a relatively high value of internal capacitance, in high frequency applications wherein common voltage transients tend to appear on the guarded side of the circuitry the transients are directly coupled into the circuitry of the unguarded side to affect threshold levels and propagation delays of digital circuitry therein. The cost of incorporating error correcting circuitry or software into the system, or of employing optical couplers having better high frequency isolation characteristics, is high.